Bottom Line:
Y. pestis recently evolved from the gastrointestinal pathogen Y. pseudotuberculosis; however, it is not known at what point Y. pestis gained the ability to induce a fulminant pneumonia.As Y. pestis further evolved, modern strains acquired a single amino-acid modification within Pla that optimizes protease activity.While this modification is unnecessary to cause pneumonic plague, the substitution is instead needed to efficiently induce the invasive infection associated with bubonic plague.

ABSTRACTYersinia pestis causes the fatal respiratory disease pneumonic plague. Y. pestis recently evolved from the gastrointestinal pathogen Y. pseudotuberculosis; however, it is not known at what point Y. pestis gained the ability to induce a fulminant pneumonia. Here we show that the acquisition of a single gene encoding the protease Pla was sufficient for the most ancestral, deeply rooted strains of Y. pestis to cause pneumonic plague, indicating that Y. pestis was primed to infect the lungs at a very early stage in its evolution. As Y. pestis further evolved, modern strains acquired a single amino-acid modification within Pla that optimizes protease activity. While this modification is unnecessary to cause pneumonic plague, the substitution is instead needed to efficiently induce the invasive infection associated with bubonic plague. These findings indicate that Y. pestis was capable of causing pneumonic plague before it evolved to optimally cause invasive infections in mammals.

Mentions:
Pla is necessary for the progression of pneumonic plague in modern lineages of Y. pestis; therefore, we tested whether the acquisition of Pla by Pestoides F was all that was required for one of the earliest existing ancestral strains of Y. pestis to cause primary pneumonic plague. On introduction to mice via the i.n. route, we found that Pestoides F carrying pPCP1 was able to grow to high levels in the lungs and cause the death of the animals within 3–3.5 days, to the same extent and rate as wild-type Y. pestis CO92 (not statistically different, Mann–Whitney U-test; Fig. 4a,b). This increase in bacterial burden within the lungs is Pla dependent in both Pestoides F and CO92, as the time to death and enumerated c.f.u. were significantly decreased in the Δpla isogenic mutant infections in both backgrounds (Fig. 4a,b). An examination of the host response to Pestoides F harbouring pPCP1 revealed large lobar pulmonary lesions (Fig. 4c), a significant increase in total cell number (Fig. 4d) with neutrophils being the primary infiltrating cell type (Supplementary Fig. 4a), and a robust inflammatory cytokine response, all which are similar to that caused by modern Y. pestis strain CO92 (Supplementary Fig. 4b). In total, our results demonstrate that the acquisition pPCP1/Pla by one of the most deeply rooted strains of Y. pestis was sufficient for this newly emerged species to cause primary pneumonic plague within rodents.

Mentions:
Pla is necessary for the progression of pneumonic plague in modern lineages of Y. pestis; therefore, we tested whether the acquisition of Pla by Pestoides F was all that was required for one of the earliest existing ancestral strains of Y. pestis to cause primary pneumonic plague. On introduction to mice via the i.n. route, we found that Pestoides F carrying pPCP1 was able to grow to high levels in the lungs and cause the death of the animals within 3–3.5 days, to the same extent and rate as wild-type Y. pestis CO92 (not statistically different, Mann–Whitney U-test; Fig. 4a,b). This increase in bacterial burden within the lungs is Pla dependent in both Pestoides F and CO92, as the time to death and enumerated c.f.u. were significantly decreased in the Δpla isogenic mutant infections in both backgrounds (Fig. 4a,b). An examination of the host response to Pestoides F harbouring pPCP1 revealed large lobar pulmonary lesions (Fig. 4c), a significant increase in total cell number (Fig. 4d) with neutrophils being the primary infiltrating cell type (Supplementary Fig. 4a), and a robust inflammatory cytokine response, all which are similar to that caused by modern Y. pestis strain CO92 (Supplementary Fig. 4b). In total, our results demonstrate that the acquisition pPCP1/Pla by one of the most deeply rooted strains of Y. pestis was sufficient for this newly emerged species to cause primary pneumonic plague within rodents.

Bottom Line:
Y. pestis recently evolved from the gastrointestinal pathogen Y. pseudotuberculosis; however, it is not known at what point Y. pestis gained the ability to induce a fulminant pneumonia.As Y. pestis further evolved, modern strains acquired a single amino-acid modification within Pla that optimizes protease activity.While this modification is unnecessary to cause pneumonic plague, the substitution is instead needed to efficiently induce the invasive infection associated with bubonic plague.

ABSTRACTYersinia pestis causes the fatal respiratory disease pneumonic plague. Y. pestis recently evolved from the gastrointestinal pathogen Y. pseudotuberculosis; however, it is not known at what point Y. pestis gained the ability to induce a fulminant pneumonia. Here we show that the acquisition of a single gene encoding the protease Pla was sufficient for the most ancestral, deeply rooted strains of Y. pestis to cause pneumonic plague, indicating that Y. pestis was primed to infect the lungs at a very early stage in its evolution. As Y. pestis further evolved, modern strains acquired a single amino-acid modification within Pla that optimizes protease activity. While this modification is unnecessary to cause pneumonic plague, the substitution is instead needed to efficiently induce the invasive infection associated with bubonic plague. These findings indicate that Y. pestis was capable of causing pneumonic plague before it evolved to optimally cause invasive infections in mammals.